1. Field of the Invention
The present invention relates to flasked instrument assemblies that are used in downhole tools, and, in particular, to the cooling of the electronic chassis in such an assembly.
2. Description of the Prior Art
It is well-known that downhole instrument assemblies are used in extremely hostile environments. Downhole tools such as logging tools, logging while drilling tools, measurement while drilling tools, and guidance tools that are used in the drilling of deviated wells employ such assemblies. Downhole instrument assemblies typically comprise thermally sensitive components which have a maximum temperature above which they will not operate properly. Such components may, for example, be electronic, optical or mechanical devices which are used to measure various parameters of the well or the formation or the fluid in the well or the fluid in the formation. In order to protect the components in these downhole instrument assemblies, the components are encased in a thermal flask.
When these downhole instrument assemblies contain electronic components, such components are mounted on an electronics chassis in the thermal flask. One function that the thermal flask provides is to isolate the electronic components from the heat of the environment in the wellbore. Such thermal flasks also contain the heat which is generated by the operation of the electronic components in the electronics chassis. The electronics chassis is designed to provide a large thermal mass, which enables the instrument assembly to operate downhole for an extended period of time before the temperature within the thermal flask become such that the operation of the electronic components are degraded. When the electronics chassis reaches a certain critical temperature, downhole operations must be stopped and the instrument assembly must be hoisted back to the surface in order to prevent damage to the assembly. Downhole operations may only be resumed once the electronics chassis has sufficiently cooled down.
The properties of the thermal flask which protect the electronics chassis from environmental heat also retard the release of the heat generated by the electronic components within the flask. Accordingly, research studies by the Assignee of this application have shown that cooling a logging tool by radiation and convection alone (i.e., passive cooling) will require a substantial amount of time, e.g., sixty plus hours, before logging operations may be resumed. Further, it is not feasible to extract the hot electronic components from the thermal flask in an effort to expedite cooling, because such extraction subjects the electronic components to thermal shock and exposure to atmospheric moisture.